An Organic Light Emitting Display (OLED), which is a current-driven display, requires a steady current to drive it to emit light. Due to the process procedure, device aging and the like, the threshold voltage (Vth) of a driving transistor in a pixel circuits is prone to drift, resulting in display unevenness and afterimages.
Currently, there are generally two ways to avoid the drift of the threshold voltage. The first way is using a pixel circuit having internal compensation to compensate the threshold voltage of the driving transistor, and the second way is using an external compensation circuit to compensate the threshold voltage of the driving transistor in the pixel circuit to alleviate the problems of display unevenness and afterimages.
However, with a pixel circuit having internal compensation, it is difficult to adjust the threshold voltage to a standard threshold voltage. Therefore, the threshold voltage inevitably may still drift. When an external compensation circuit is used, a common method is to add a variation of the threshold voltage to a data signal to realize the compensation of the threshold voltage. However, this method is far from alleviating the drift of the threshold voltage but worsens the drift of the threshold voltage; for example, when the threshold voltage drifts positively under a positive bias, if the increasing threshold voltage is added to a data signal through an external compensation circuit, the positive bias may become larger, causing more serious positive drift of the threshold voltage and therefore worsening the afterimage problem.